1. Field of the Invention
The present invention relates to a method of continuously producing molded members, especially sheets, from a mixture of gypsum and fibrous material. In a first mixing stage, respective measured-out quantities of gypsum and fibrous material are thoroughly mixed together, with the dry gypsum/fibrous material mixture being weighed and subsequently moistened in second mixing stage by supplying water thereto in conformity to the weight of the dry mixture. The moistened mixture is subsequently dispersed onto a support to form molded members, with the surface of each molded member being additionally moistened, and the latter then being compacted. The present invention also relates to an apparatus for carrying out such a method. The apparatus includes: a first metering mechanism for the measured-out addition of gypsum and fibers to a dry mixer; a second metering mechanism for measuring out a given volume and/or weight of dry gypsum/fiber mixture, as a function of which a third metering mechanism for measuring out liquid is controlled; a wetting mixer for receiving dry gypsum/fiber mixture from the second metering mechanism, and liquid from the third metering mechanism for moistening the mixture; a dispersal mechanism, the dispersal head of which disperses moistened gypsum/fiber mixture from the wetting mixer as a molded member onto a forming line; and spray nozzles disposed, when viewed in the conveying direction of the forming line, before and after the dispersal head for additionally moistening the surface of the molded member.
2. Description of the Prior Art
A method of the aforementioned general type is known from German Offenlegungsschrift 34 04 658, published Aug. 14, 1985 belonging to the assignee of the present invention. The dry, pre-mixed gypsum/fiber mass is moistened in a further mixing stage by adding a measured quantity of water thereto. In so doing, the maximum amount of water added is that quantity which is stoichiometrically required for assuring curing of the gypsum. By preparing the mixture in this fashion, the material which is to be dispersed can be transported and stored with no problem. The instruments used do not become very dirty, and it is easy to separate and orient the fibers. It is even possible to eliminate drying if water is supplied in a precisely stoichiometric quantity.
However, practice has shown that an exact stoichiometric supply of water is generally impossible, since slight variations in the mixing ratio and in the quality of the raw materials invariably occur. If an excess amount of water is added, it is necessary to subsequently dry the sheets, whereas if too little water is added there is not enough water for the complete curing reaction of the gypsum, which leads to a considerable weakening of the sheets. Furthermore, when water is added in stoichiometric quantities, between 1 and 3% of residual moisture remains. This unused residual moisture corresponds to a quantity of 5 to 15% non-cured gypsum, which leads to a weakening of the strucure of the sheets.
The remaining 1 to 3% residual moisture is a drawback in that it is practically impossible to use such sheets as high-grade construction material. It is precisely in the range of 0 to 3% residual moisture that the gypsum/fibrous material experiences very great changes in volume, which can show up in an up to 0.3 of the % linear change in lengths (3 mm per m). If in the extreme case moist sheets that have been placed so that no joint appears are dried to 0% residual moisture, all of the joints pull apart during subsequent adjustment to the ambient temperature.
Furthermore, the course of the curing process is disadvantageous when a stoichiometric quantity of water is added. The gypsum crystallizes on the spot because no water is available for the transport of the calcium sulfate ions. The sheet therefore becomes an easily caking aggregate of granules that retain the original shape of the gypsum granules.
Although during a subsequent spraying of the surface of the molded member with water this surface receives a firm, uniform structure, it has been proven that the stabilization or setting of the surface by itself does not assure a sufficiently high sheet quality. In particular, when the sheets are nailed or screwed, large chunks of the solidified surface break off.
German Offenlegungsschrift 34 04 658 dated Aug. 14, 1985 belonging to the assignee of the present invention makes reference to another German Offenlegungsschrift 32 16 886 dated Nov. 10, 1983 and also belonging to the assignee of the present invention. This known method allows plaster-fiber plates, sheets or panels of satisfactory quality to be produced. However the disadvantage exists that during mass production of such plaster-fiber plates, sheets or panels, the statistical dispersal for transverse stress and flexure, transverse strength or bending strength is comparatively great. Also there has been shown that smaller, mostly point-shaped provision of fiber nests upon the panel outer side can lead to small pock-type elevations, which for many applications or purposes of use represent an influencing of the quality of such plaster-fiber panels and consequently make necessary eventually a grinding or post-working of the surface thereof.
An object of the present invention is to provide a method for the continuous production of molded members, especially sheets, from a mixture of gypsum and fibrous material, with the sheets produced pursuant to this method receiving a pervading high strength with little variation, with this strength being obtained in a short curing and drying time.
A further object of the present invention is to provide a method for continuous production of molded bodies or members, especially of plates, sheets, or panels, of plaster or calcined gypsum and fiber material, with which the surfaces of the plates or panels in essence are free of pocks or depressions and flaws and with which furthermore optimum strength values of nominal statistical dispersal also are attainable with mass production. Furthermore an apparatus for performing and carrying out this inventive method can be found disclosed herewith.
It is a further object of the present invention to provide an apparatus for carrying out the aforementioned method.